Oxygen Background Pressure Research Articles

Effects of temperature, target/substrate distance, and background pressure on growth of ZnO nanorods by pulsed laser deposition.

The growth of ZnO nanorods on the c-plane of Al2O3 substrates by PLD was been investigated by controlling processing conditions such as growth temperature, distance between target and substrate, and background oxygen pressure. ZnO nanorods were observed from the growth temperature of 600 degrees C for the oxygen pressure of 30 mTorr and the target/substrate distance of 70 mm. The diameters of the ZnO nanorods at the temperature of 700 degrees C and the oxygen pressure of 30 mTorr were approximately 200, 70, and 40 nm for the distance of 45, 70, and 100 mm, respectively. ZnO films without nanorods were observed at the distance of 70 mm and the temperature of 700 degrees C when the oxygen pressure decreased to 1 mTorr. The kinetic energy of the ablated particles by the laser decreases during collisions with background oxygen molecules, resulting in conditions that favor the growth of ZnO nanorods.

Deposition of low sheet resistance indium tin oxide directly onto functional small molecules

We outline a methodology for depositing tin-doped indium oxide (ITO) directly onto semiconducting organic small molecule films for use as a transparent conducting oxide top-electrode. ITO films were grown using pulsed laser deposition onto copper(II)phthalocyanine (CuPc):buckminsterfullerene (C60) coated substrates. The ITO was deposited at a substrate temperature of 150°C over a wide range of background oxygen pressures (Pd) (0.67–10Pa). Deposition at 0.67≤Pd≤4.7Pa led to delamination of the organic films owing to damage induced by the high energy ablated particles, at intermediate 4.7≤Pd<6.7Pa pressures macroscopic cracking is observed in the ITO. Increasing Pd further, ≥6.7Pa, supports the deposition of continuous, polycrystalline and highly transparent ITO films without damage to the CuPc:C60. The free carrier concentration of ITO is strongly influenced by Pd; hence growth at >6.7Pa induces a significant decrease in conductivity; with a minimum sheet resistance (Rs) of 145Ω/□ achieved for 300nm thick ITO films. To reduce the Rs a multi-pressure deposition was implemented, resulting in the formation of polycrystalline, highly transparent ITO with an Rs of ~20Ω/□ whilst maintaining the inherent functionality and integrity of the small molecule substrate.

Strained ferroelectric NaNbO3 thin films: Impact of pulsed laser deposition growth conditions on structural properties

Pulsed laser deposition (PLD) was employed to grow coherent NaNbO3 thin films on several lattice matched substrates. Compressive (tensile) lattice strain was induced by the use of NdGaO3 and SrTiO3 (DyScO3 and TbScO3) single crystals. Deposition from a stoichiometric target at a 5Pa O2 pressure was found to result in sodium and oxygen deficient films. By increasing the oxygen background pressure up to 200Pa the oxygen and sodium concentrations in films on all substrates could be increased. As an alternative way to enhance the compositional ordering in the films sodium excess ceramics were used as PLD targets. With a Na/Nb ratio up to 1.17, phase pure, well-ordered films with smooth surfaces were grown on all substrates. The film composition was found to have a crucial impact on the formation of ferroelectric domains in tensilely strained films on DyScO3 substrates. Regularly arranged ferroelectric stripe domains were obtained at an oxygen partial pressure of 5Pa using sodium rich growth parameters via the application of a 1.17 Na/Nb ratio PLD target.

Influence of La and Mn vacancies on the electronic and magnetic properties ofLaMnO3thin films grown by pulsed laser deposition

With pulsed laser deposition we have grown c-axis oriented thin films of the nominal composition LaMnO$_{3}$ (LMO) on LSAT(001) substrates. We find that, depending on the oxygen background pressure during growth, the LMO films contain sizeable amounts of La and/or Mn vacancies that strongly influence their electronic and magnetic properties. Specifically, we show that the Mn/La ratio can be systematically varied from 0.92 at 0.11 mbar to 1.09 at 0.30 mbar of oxygen. These cationic vacancies lead to markedly different disorder effects that become most pronounced once the samples are fully oxygenated and thus strongly hole doped. All as-grown and thus slightly oxygen deficient LMO films are ferromagnetic insulators with saturation moments in excess of 2.5 \mu $_{B}$ per Mn ion, their transport and optical properties that can be understood in terms of trapped ferromagnetic polarons. Upon oxygen annealing, the most La-deficient films develop a metallic response with an even larger ferromagnetic saturation moment of 3.8 \mu $_{B}$ per Mn ion. In contrast, in the oxygenated Mn-deficient films the ferromagnetic order is almost completely suppressed to less than 0.5 \mu $_{B}$ per Mn ion and the transport remains insulator-like. We compare our results with the ones that were previously obtained on bulk samples and present an interpretation in terms of the much stronger disorder potential of the Mn vacancies as compared to the La vacancies. We also discuss the implications for the growth of LMO thin films with well-defined physical properties that, for example, are a prerequisite for the study of interface effects in multilayers.

Influence of oxygen pressure and aging on LaAlO3 films grown by pulsed laser deposition on SrTiO3 substrates

The crystal structures of LaAlO3 films grown by pulsed laser deposition on SrTiO3 substrates at oxygen pressure of 10-3 mbar or 10-5 mbar, where kinetics of ablated species hardly depend on oxygen background pressure, are compared. Our results show that the interface between LaAlO3 and SrTiO3 is sharper when the oxygen pressure is lower. Over time, the formation of various crystalline phases is observed while the crystalline thickness of the LaAlO3 layer remains unchanged. X-ray scattering as well as atomic force microscopy measurements indicate three-dimensional growth of such phases, which appear to be fed from an amorphous capping layer present in as-grown samples.

Influence of background oxygen pressure on film properties of pulsed laser deposited Y:BaZrO3

Yttrium-doped barium zirconate (Y:BaZrO3, BZY) thin films were prepared using pulsed laser deposition (PLD) on MgO substrates at 700°C by varying the ambient oxygen pressure. The microstructure and electrical properties were then investigated using transmission electron microscopy and electrochemical impedance spectroscopy. Dense, well-crystallized BZY films were produced at ambient oxygen pressures (Pamb,O2) of ≤6.67Pa. At Pamb,O2≥13.3Pa, conical amorphous defects and porous microstructures were observed in the films. The conductivity of PLD BZY prepared at Pamb,O2≤13.3Pa was measured to be close to that of reference polycrystalline BZY films, and the grain-boundary properties governed the overall ion conduction, with nano-grains being covered by the space charge depletion zones. At Pamb,O2≥13.3Pa, microstructural defects significantly degraded the electrical properties.

Growth and optical characterization of diamond-shaped zinc oxide nanostructures deposited by pulsed laser ablation

We report on the growth of zinc oxide diamond-shaped nanostructured thin films by pulsed laser deposition technique on silicon substrate at different substrate temperatures (room temperature to 600 \(^{\circ }\)C) and at fixed background pressure of oxygen using Nd:YAG laser at a wavelength of 532 nm. The influence of substrate temperature on the grain size, surface morphology and optical properties is characterized by x-ray diffraction, field-emission scanning electron microscope, photoluminescence and Raman spectroscopy. x-ray diffraction results show that the grain size increases with increasing substrate temperature during the growth of ZnO thin films due to improved crystallinity but at 450 \(^{\circ }\)C the crystallinity degrades. It is attributed to the formation of diamond-shaped ZnO nanostructures as supported by the field emission scanning electron microscope images. Consequently, increase in photoluminescence and Raman intensities is also attributed to the formation of diamond-shaped structures. The growth of diamond-shaped structure is discussed in the light of growth of various planes in the hexagonal structure of ZnO.

Novel silicon surface passivation by porous silicon combined with an ultrathin Al2O3 film

We report on a dual passivation approach of monocrystalline-silicon (c-Si), which combines porous silicon (PS) treatment and pulsed laser deposition (PLD) of Al2O3 ultrathin layer. Al2O3 ultrathin films were deposited, under an oxygen background pressure of 10 mTorr. In this work we demonstrate that the dual treatment Al2O3/PS provide excellent passivation quality of c-Si surfaces. Surface passivation and reflectivity properties are investigated before and after heat treatment at 700 °C. The level of surface passivation is quantified using photoconductance and Fourier-transform infra-red absorption techniques. X-ray diffraction analysis suggested that the heat treatment leads to an average crystallite size decrease which help the diffusion of the Al2O3 NPs along the wafer surface. As a result, the effective minority carrier lifetime increases from 2 to 7 μs. Our results demonstrate that this dual treatment not only provides strong passivation of the c-Si substrate but decreases also the total surface reflectivity at 500 nm (from 28 % for untreated c-Si samples to ∼7 % for Al2O3/PS treated ones).

Optical properties and thermal stability of alumina films grown by pulsed laser deposition

Thin films of aluminum oxide of thickness 293–433 nm were grown on fused silica substrates by pulsed laser deposition (PLD) of an alumina target with KrF excimer laser radiation. The effects of oxygen background pressure (0.5–50 Pa) and substrate temperature of room temperature to 800 °C were investigated on thickness, crystal-structure, optical properties and thermal stability of alumina films. It was found that PLD grows a weakly crystalline aluminum oxide film at in situ substrate temperature of 800 °C and oxygen background pressure of 5 Pa. Post-deposition annealing studies of amorphous alumina sample prepared at in situ temperature of 600 °C found that crystallization starts at 800 °C and forms δ alumina phase.

Smooth MgO films grown on graphite and graphene by pulsed laser deposition

Pulsed laser deposition was used to grow thin (1–100 nm) magnesium oxide films directly on graphite and epitaxial graphene on SiC(0001). The authors observe very smooth (typical rms roughness of ∼0.4 nm) film morphologies that are nearly independent of film thickness and conformal to the substrate for films grown on room temperature substrates. Surface roughness is less than 1 nm for thicknesses up to 100 nm and is independent of oxygen background pressure during growth. X-ray diffraction shows no evidence of crystallinity for films grown on room temperature substrates but shows ⟨100⟩ texture for films grown on heated substrates that also have very rough surface morphologies. X-ray photoelectron spectroscopy shows hydroxylation of films due to air exposure that can only be partially removed by annealing, indicating the presence of atomic defects in the films.

SIMS of thin films grown by pulsed laser deposition on isotopically labeled substrates

Abstract The oxygen diffusion kinetics in a strontium titanate (STO)–yttria stabilized zirconia (YSZ) heterostructure were investigated by secondary ion mass spectrometry (SIMS) depth profiling of pulsed laser deposited thin films grown on 18 O enriched single crystalline substrates. By annealing for 100 h at 1100 °C in an 18 O 2 enriched atmosphere 18 O concentrations of ~ 81% and ~ 63% were reached in the YSZ, and STO substrates, respectively. The film depositions were performed at different substrate temperatures up to 750 °C and different oxygen background pressures in a range from 1.5 × 10 − 5 mbar to 0.1 mbar to observe differences in the diffusion of the oxygen ions from the substrate into the films. Flat profiles of 18 O were obtained for the YSZ thin films implying a very fast diffusion that determines the isotope distribution during the deposition. For the STO films pronounced concentration profiles were obtained at 650 °C allowing estimations of the diffusion constant. Elemental mapping of the YSZ films revealed pinholes of micron size which are hardly detectable by other techniques.

Critical influence of target-to-substrate distance on conductive properties of LaGaO3/SrTiO3 interfaces deposited at 10−1 mbar oxygen pressure

We investigate pulsed laser deposition of LaGaO3/SrTiO3 at 10−1 mbar oxygen background pressure, demonstrating the critical effect of the target-to-substrate distance, dTS, on the interface sheet resistance, Rs. The interface turns from insulating to metallic by progressively decreasing dTS. The analysis of the LaGaO3 plume evidences the important role of the plume propagation dynamics on the interface properties. These results demonstrate the growth of conducting interfaces at an oxygen pressure of 10−1 mbar, an experimental condition where a well-oxygenated heterostructures with a reduced content of oxygen defects is expected.

Enhancement in optical and microstructure properties of Er3+-doped phospho-tellurite glass thin film

Surface structure and photoluminescence lifetime of Er3+:4I13/2→4I15/2 optical transition are two main factors determining the potential of glass thin films for integrated optical waveguide amplifiers. In this paper, the effects of different ambient gas atmospheres, i.e. pure O2 and O2–He gas mixture, on the surface structure of the phospho-tellurite (PT) glass thin films which have been grown by the technique of pulsed laser deposition was studied. The surface analysis of the deposited thin films in the presence of the O2–He gas mixture showed a smoother surface and smaller particle size compared to the thin films deposited using a low background pressure of pure oxygen, which yields a reduction in the measure propagation loss from 0.78 to 0.41dB/cm.It was found that the deposition of a thin layer of silica on top of the PT glass thin films increased the measured lifetime of the Er3+:4I13/2→4I15/2 optical transition, as a result of the reduction in post deposition OH absorption by more than ∼500cm−1.

PLD and RF discharge combination used for preparation of photocatalytic TiO2 layers

Utilizing pulsed laser deposition technique combined with the radio-frequency discharge (between the target and the substrate), we were able to grow polycrystalline titanium dioxide layers (anatase, rutile) at substrate temperatures 85°C and 150°C. Besides the discharge no additional substrate heating was applied. The layers were prepared from pure titanium and titanium dioxide targets. To optimize deposition conditions, the oxygen background pressure, fluence (from 2Jcm−2 to 9Jcm−2), and discharge power were varied. Silicon wafers (111), fused silica, and polyethylene tubes were used as substrates. The layers’ crystalline structure was determined by X-ray diffraction. Atomic force microscopy was used to characterize the surface properties. Two methods were implemented to determine TiO2 photocatalytic characteristics: 4-chlorphenol solution degradation evaluated by pH measurement and the decomposition of Oleic acid evaluated by the change of the water contact angle on the surface layer. Both methods indicated similar results: the highest photoactivity was observed on the layers consisting from anatase phase and mixture of anatase and rutile.

Vacuum ultraviolet frequency combs generated by a femtosecond enhancement cavity in the visible

We present the first (to our best knowledge) femtosecond enhancement cavity in the visible wavelength range for ultraviolet frequency comb generation. The cavity is seeded at 518 nm by a frequency-doubled Yb fiber laser and operates at a peak intensity of 1.2×10(13) W/cm(2). High harmonics of up to the ninth order (~57 nm) are generated in an intracavity xenon gas jet. Intracavity high harmonic powers of several milliwatts for the third harmonic order and microwatts for the fifth harmonic order prove the potential of the "green cavity" as an efficient ultraviolet frequency comb source for future spectroscopic experiments. A limiting degradation effect of the cavity mirrors is avoided by operating at a constant oxygen background pressure.

Nanoforest Nb2O5 Photoanodes for Dye-Sensitized Solar Cells by Pulsed Laser Deposition

Vertically aligned bundles of Nb(2)O(5) nanocrystals were fabricated by pulsed laser deposition (PLD) and tested as a photoanode material in dye-sensitized solar cells (DSSC). They were characterized using scanning and transmission electron microscopies, optical absorption spectroscopy (UV-vis), and incident-photon-to-current efficiency (IPCE) experiments. The background gas composition and the thickness of the films were varied to determine the influence of those parameters in the photoanode behavior. An optimal background pressure of oxygen during deposition was found to produce a photoanode structure that both achieves high dye loading and enhanced photoelectrochemical performance. For optimal structures, IPCE values up to 40% and APCE values around 90% were obtained with the N(3) dye and I(3)(-)/I(-) couple in acetonitrile with open circuit voltage of 0.71 V and 2.41% power conversion efficiency.

Oxygen background gas influence on pulsed laser deposition process of LaAlO3 and LaGaO3

We investigate pulsed laser ablation of LaAlO3 and LaGaO3 with a focus on the influence of oxygen background gas pressure on the plume expansion dynamics and deposition rate. The ablation plume is characterized by exploiting fast photography and time- and space-resolved optical emission spectroscopy. The variation of the deposition rate with the oxygen background pressure was obtained at 800°C by reflection high-energy electron diffraction, and compared to that measured at room temperature by means of a quartz crystal microbalance. The experimental findings allow one to address the various stages of plume expansion as a function of the background oxygen pressure as well as the changes induced on the plume species kinetic energy and composition. On the base of our experimental results, the possible influence of various mechanisms, such as subplantation and oxygen vacancy formation, on the growth of oxides interfaces is addressed.

Tailoring of LaxSr1‐xCoyFe1‐yO3‐δ Nanostructure by Pulsed Laser Deposition

AbstractPulsed Laser Deposition (PLD) was used to prepare thin films with the nominal composition La0.58Sr0.4Co0.2Fe0.8O3‐δ (LSCF). The thin film microstructure was investigated as a function of PLD deposition parameters such as: substrate temperature, ambient gas pressure, target‐to‐substrate distance, laser fluence and frequency. It was found that the ambient gas pressure and the substrate temperature are the key PLD process parameters determining the thin film micro‐ and nanostructure. A map of the LSCF film nanostructures is presented as a function of substrate temperature (25–700 °C) and oxygen background pressure (0.013–0.4 mbar), with film structures ranging from fully dense to highly porous. Fully crystalline, dense, and crack‐free LSCF films with a thickness of 300 nm were obtained at an oxygen pressure lower than 0.13 mbar at a temperature of 600 °C. The obtained knowledge on the structure allows for tailoring of perovskite thin film nanostructure, e.g., for solid oxide fuel cell cathodes. A simple geometrical model is proposed, allowing estimation of the catalytic active surface area of the prepared thin films. It is shown that voids at columnar grain boundaries can result in an increase of the surface area by approximately 25 times, when compared to dense flat films.

Electronic structure of Co2+ions in anatase Co:TiO2in relation to heterogeneity and structural defects

We have studied the electronic structure of Co ions in semiconducting anatase Co:TiO2(1.4 at.% Co) with x-ray-absorption spectroscopy. The density of structural defects is varied by (i) the oxygen vacancy concentration depending on the oxygen background pressure during growth, and (ii) the film thickness. TheCoL2,3-edge spectra, which are characteristic of Co2+ (3d7) ions, show particularly pronounced ligand-to-metal charge-transfer (CT) satellites for the thickest oxygen-poor films, which also exhibit the highest structural defect density. With charge-transfer multiplet calculations, the CT satellites are modeled by configuration interaction, originating from hybridization between Co 3d- and ligand states. This hybridization adds to the complex interplay between Co heterogeneity, defects, and electronic and magnetic properties in Co:TiO2.

Zinc oxide nanostructured layers for gas sensing applications

Various kinds of zinc oxide (ZnO) nanostructures, such as columns, pencils, hexagonal pyramids, hexagonal hierarchical structures, as well as smooth and rough films, were grown by pulsed laser deposition using KrF and ArF excimer lasers, without use of any catalyst. ZnO films were deposited at substrate temperatures from 500 to 700°C and oxygen background pressures of 1, 5, 50, and 100 Pa. Quite different morphologies of the deposited films were observed using scanning electron microscopy when different laser wavelengths (248 or 193 nm) were used to ablate the bulk ZnO target. Photoluminescence studies were performed at different temperatures (down to 7 K). The gas sensing properties of the different nanostructures were tested against low concentrations of NO2. The variation in the photoluminescence emission of the films when exposed to NO2 was used as transduction mechanism to reveal the presence of the gas. The nanostructured films with higher surface-to-volume ratio and higher total surface available for gas adsorption presented higher responses, detecting NO2 concentrations down to 3 ppm at room temperature.